Transit-time difference type ultrasonic flowmeter

A transit-time difference type ultrasonic flowmeter is disclosed comprising: (a) a pair of ultrasonic transducers (2a, 2b) mounted on an outer surface of a flow tube (1) at an upstream region and a downstream region thereof respectively, a switching device (3) for switching the operational mode of each of the pair of transducers (2a; 2b) alternatively to its transmitting or receiving mode, an amplifier (5) for amplifying a signal representing ultrasonic waves propagating through fluid received by the ultrasonic transducer of the receiving side, and data reduction equipment (8) for processing the amplified received signal to output a flow rate signal; (b) the data reduction equipment (8) includes an analog-digital converter (9) and a digital signal processor (10); (c) the analog-digital converter (9) converts the waveform of the received signal amplified by the amplifier (5) into a plurality of voltage-time data sets; and (d) the digital signal processor (10) picks up, on the basis of the data of time, an object peak (P 1 ) from the waveform of the voltage-time data sets, estimates at least one zero-cross point (q 1 , q 2 ...) confined by the object peak (P 1 ) or a peak (P 2 ) adjacent to the object peak through the calculation made on the plurality of voltage-time data sets distributed along the time axis close to the zero-cross points (q 1 , q 2 ...) and finds the time required for propagating the ultrasonic wave from one zero-cross point or the average time required for propagating the ultrasonic wave from a plurality of zero-cross points, wherein the digital signal processor (10) measures the height of the object peak (P' max ) and estimates the amount of attenuation of the received signal due to the impurities such as bubbles or solid particles included in the fluid, and wherein if the amount of attenuation (P max /P' max ) is larger than a predetermined value, the calculation for estimating the zero-cross points (q 1 ...) is suspended, and the flow rate signal is output by adopting the measured value of time required for propagation or the output value of flow rate obtained on the last measurement of the repeatedly effected measurements.